1. Field of the Invention
The present invention relates to the field of mobile communications systems. More particularly, the present invention relates to the calculation of inter-cell interference in a cellular mobile communications system.
Universal Mobile Telecommunication Services (UTMS) are being developed, in which voice and data signals are transmitted using a Wideband Code Division Multiple Access (WCDMA) transmission method. One or more services may be provided in the network, for example, a voice-only service, a data-only service, and a mixed voice/data service. In general, UTMS networks are cellular networks based on a system of hexagonal regions each having its own base station for handling transmission to/from mobile communications units (mobiles) in the cells of that region. Typically, each hexagonal region is divided into three sectors (cells) and has six nearest neighbors (see FIG. 1).
The maximum capacity of a WCDMA system is determined, at least in part, by the interference that occurs at a base station's antenna between signals received from mobiles in the cell of interest and signals received from mobiles in other cells. Accordingly, it is useful to be able to evaluate this forward-link, or uplink, inter-cell interference in order to be able to estimate system capacity (as well as other parameters). (Of course, uplink intra-cell interference also affects the studied cell, but the present invention is primarily concerned with evaluation of uplink inter-cell interference).
2. Description of the Related Art
Typically, uplink inter-cell interference is calculated at a base station BS0 for an antenna serving one of its sectors, cell C0, and is equal to the sum of received powers (that is power received at this base station antenna) coming from mobiles of other cells C1, C2, . . . Cq. Usually, calculations of uplink inter-cell interference assume that interference comes from cells served by base stations arranged in two rings around the base station serving the cell of interest (interfering cells of only one ring of “nearest-neighbor” base stations are shown in FIG. 1). Up to now, three different approaches have been adopted for evaluating uplink inter-cell interference.
A first known approach assumes that uplink inter-cell interference will be some fraction, f, of the intra-cell interference (this intra-cell interference is the sum of all received powers coming from other mobiles in the same cell as the mobile of interest). In order to find a value to assign to this fraction, f, either analytical studies or system simulations are performed.
Because this first approach requires as an input a value for the fraction, f, it does not allow inter-cell interference to be evaluated independently of previous computations. Moreover, a value for the fraction, f, is only known for a limited number of system configurations.
A second known approach computes uplink inter-cell interference based on system simulations. These require simulation not only of the mobiles within the cell of interest (and their power values) but also of the mobiles in neighboring cells which may generate an appreciable amount of interference.
Because this second known approach requires modeling of large numbers of mobiles, the computation complexity is very high, especially when considering power control.
A third known approach simplifies calculations by treating all mobiles of a given service within a cell as contributing to a uniform distribution of traffic over the cell, rather than treating each mobile as a discrete unit. Uplink inter-cell interference is then determined by numerical integration over the surface of the cells neighboring the cell of interest.
This third approach is based on an assumption that all mobiles for one service are received with the same power in their respective cell. However, in reality mobiles have limited range. Moreover, the complexity of this evaluation method increases with the number of services being taken into account simultaneously.